Japanese Patent Application No. 2000-244747, filed on Aug. 11, 2000, is hereby incorporated by reference in its entirety.
The present invention relates to a light-emitting device using electroluminescence (EL).
A semiconductor laser is used as a light source for optical communications systems. The semiconductor laser excels in wavelength selectivity and is capable of emitting light in a single mode. However, it is difficult to fabricate a semiconductor laser because many stages of crystal growth are needed. Moreover, since types of light-emitting materials used for the semiconductor laser are limited, the semiconductor laser cannot emit light with various wavelengths.
Conventional EL light-emitting devices emit light with a broad spectral width and are used in some applications such as for displays. However, conventional EL light-emitting devices are unsuitable for optical communications and the like in which light with a narrow spectral width is required.
An objective of the present invention is to provide a light-emitting device which can emit light with a remarkably narrow spectral width in comparison with conventional EL light-emitting devices and with directivity, and is applicable not only to displays but also to optical communications and the like.
First Light-emitting Device
A first light-emitting device according to a first aspect of the present invention comprises:
a light-emitting layer which emits light by electroluminescence;
a pair of electrode layers for applying an electric field to the light-emitting layer; and
an optical element for causing light generated in the light-emitting layer to be transmitted in a predetermined direction,
wherein the optical element forms an incomplete photonic band which inhibits spontaneous emission of light in one dimension or two dimensions; and
wherein light generated in the light-emitting layer is emitted by inhibiting spontaneous emission in two dimensions.
The incomplete photonic band used herein refers to a band formed in the case where a complete photonic band gap is not formed. For example, in the case where the optical element is in a shape of grating in which a first medium layer and a second medium layer are arranged alternately, a complete photonic band gap may not be formed when the difference in the refractive indices between the first medium layer and the second medium layer is small.
According to the first light-emitting device, electrons and holes are injected into the light-emitting layer respectively from the pair of electrode layers, specifically, a cathode and an anode. Light is emitted when the molecules return to the ground state from the excited state by allowing the electrons and holes to recombine in the light-emitting layer. Spontaneous emission of this light is inhibited in two dimensions, whereby the light has a very narrow spectral width and high efficiency.
Specifically, in the first light-emitting device, a photonic band is formed by the optical element. In this band, a high density of states is obtained at energy at a specific band edge. If the optical element is formed so that the energy level of the light spectrum emitted in the light-emitting layer includes the energy level at this band edge, emission of light in the light-emitting layer tends to occur at the energy level at this band edge. Therefore, the first light-emitting device is capable of emitting light with a wavelength corresponding to the energy level at a predetermined band edge and with a narrow spectral width, thereby exhibiting high emission efficiency.
Second Light-emitting Device
A second light-emitting device according to a second aspect of the present invention comprises a substrate and a light-emitting section,
wherein the light-emitting section includes:
a light-emitting layer which emits light by electroluminescence;
a pair of electrode layers for applying an electric field to the light-emitting layer;
an optical element for causing light generated in the light-emitting layer to be transmitted in a predetermined direction; and
an insulating layer which is disposed between the pair of electrode layers, partially has an opening through which current is supplied to the light-emitting layer, and functions as a current blocking layer which determines a region in which current flows,
wherein the optical element forms an incomplete photonic band which inhibits spontaneous emission of light in one dimension or two dimensions; and
wherein light generated in the light-emitting layer is emitted by inhibiting spontaneous emission in two dimensions.
According to the second light-emitting device, since the insulating layer in the light-emitting section functions as a current blocking layer in addition to the effects of the first light-emitting device, the region for current supplied to the light-emitting layer can be specified. Therefore, current intensity and current distribution can be controlled in the region from which it is desired to emit light, whereby light can be emitted with high emission efficiency.
In the case where the insulating layer functions as cladding, in the case of a waveguide including a light-emitting layer as a core and an insulating layer as cladding, the waveguide mode of light transmitted to the waveguide section through a light-transmitting section can be controlled by specifying the opening in the insulating layer. Specifically, the waveguide mode of light transmitted through the light-emitting layer (core) can be set at a predetermined value by specifying the width of the region in which light is confined (the width perpendicular to the direction in which light is transmitted) by the insulating layer (cladding). The waveguide mode and the waveguide generally have a relation represented by the following equation.
Nmax+1xe2x89xa7K0xc2x7axc2x7(n12xe2x88x92n22)xc2xd/(xcfx80/2)
K0: 2xcfx80/xcex
a: half width of waveguide core
n1: refractive index of waveguide core
n2: refractive index of waveguide cladding
Nmax: maximum value of possible waveguide mode
Therefore, in the case where the parameters of the above equation such as the refractive indices of the core and cladding are specified, the width of the light-emitting layer (core) specified by the width of the opening in the current blocking layer is selected depending on the desired waveguide mode. Specifically, the width (2a) of the light-emitting layer corresponding to the core in a desired waveguide mode can be calculated from the above equation by substituting the refractive indices of the light-emitting layer provided in the current blocking layer and the insulating layer as a current blocking layer for the refractive indices of the core and cladding of the waveguide, respectively. It is preferable to determine the width of the core layer in the waveguide section to which light is supplied from the light-emitting section while taking into consideration the width of the light-emitting layer determined as described above, the calculated value obtained from the above equation based on the desired waveguide mode, and the like. Light in a desired mode is transmitted from the light-emitting section to the waveguide section with high connective efficiency by setting the width of the light-emitting layer, the width of the core layer, and the like to optimum values. In the light-emitting section, there may be a case where the light-emitting layer in the current blocking layer formed by the insulating layer does not uniformly emit light. Therefore, it is preferable that the designed values for each section such as the light-emitting layer, light-transmitting section, and waveguide section be suitably adjusted based on the width (2a) of the core (light-emitting layer) calculated using the above equation so that each section exhibits high connective efficiency.
The waveguide mode of the light-emitting device is preferably 0 to 1000. In particular, the waveguide mode is preferably about 0 to 10 in communication applications. Light with a predetermined waveguide mode can be efficiently obtained by specifying the waveguide mode of light in the light-emitting layer in this manner.
Third Light-emitting Device
A third light-emitting device according to a third aspect of the present invention comprises a substrate, a light-emitting section and a waveguide section which transmits light from the light-emitting section, the light-emitting and waveguide sections being integrally formed on the substrate,
wherein the light-emitting section includes:
a light-emitting layer which emits light by electroluminescence;
a pair of electrode layers for applying an electric field to the light-emitting layer;
an optical element for causing light generated in the light-emitting layer to be transmitted in a predetermined direction; and
an insulating layer which is disposed between the pair of electrode layers and functions as a cladding layer,
wherein the waveguide section includes:
a core layer which is integrally continuous with at least part of the optical element; and
a cladding layer optically continuous with the insulating layer,
wherein the optical element forms an incomplete photonic band which inhibits spontaneous emission of light in one dimension or two dimensions, and
wherein light generated in the light-emitting layer is emitted by inhibiting spontaneous emission in two dimensions.
In addition to the effects of the first light-emitting device, according to the third light-emitting device, at least part of the optical element in the light-emitting section is integrally continuous with the core layer in the waveguide section, and the insulating layer (cladding layer) in the light-emitting section is integrally continuous with the cladding layer in the waveguide section, so that the light-emitting section and the waveguide section are optically combined with high connective efficiency, thereby enabling efficient transmission of light.
In this configuration, a material which functions as a cladding layer for the optical element is selected as the material for the insulating layer. According to this light-emitting device, since at least part of the optical element in the light-emitting section and the core layer in the waveguide section can be deposited and patterned in the same steps, the fabrication can be simplified. Moreover, the insulating layer (cladding layer) in the light-emitting section and the cladding layer in the waveguide section can be deposited and patterned in the same steps. This also simplifies the fabrication.
According to the present invention, a light-emitting device including an optical element which forms an incomplete photonic band in one dimension or two dimensions which can emit light with a remarkably narrow spectral width in comparison with conventional EL light-emitting devices and exhibiting directivity, and can be applied not only to displays but also to optical communications and the like, can be provided.
In the second and third light-emitting devices, the opening formed in the insulating layer which functions as a current blocking layer and a cladding layer may be in the shape of a slit extending in the periodic direction of the optical element, specifically, in the direction in which light is waveguided. At least part of the light-emitting layer may be present in the opening formed in the insulating layer. According to this configuration, the region of the light-emitting layer to which it is desired to supply current and the region specified by the current blocking layer can be positioned self-alignably.
In the second and third light-emitting devices, the optical element has only to have a periodic refractive index distribution in one dimension or two dimensions and forms an incomplete photonic band in one dimension or two dimensions. The optical element may have a structure such as a grating-shaped structure, a multilayer film structure, a columnar-shaped structure, or a combination of these structures. Followings are examples of the optical element.
(A) The optical element may have a refractive index distribution which is periodic in one direction in the XY surface and include first and second medium layers arranged alternately. In the case of using such an optical element, the light-emitting device according to the present invention may comprise another optical element which forms an incomplete photonic band capable of inhibiting spontaneous emission of light in two dimensions in combination with the incomplete photonic band in one dimension formed by the above optical element. The other optical element has only to inhibit transmission of light at least in the Z direction. For example, a photonic crystal capable of forming either a complete photonic band gap or incomplete photonic band gap or an optical layer such as a cladding layer or a dielectric multilayer mirror may be used. Light with a very narrow spectral width of which the spontaneous emission is inhibited in two dimensions by the optical element and the other optical element can be obtained with high efficiency.
(B) The optical element may have in the XY surface a refractive index distribution which is periodic in first and second directions and include column-shaped first medium layers arranged in the shape of a tetragonal lattice and a second medium layer formed between the first medium layers. This optical element can form an incomplete photonic band in which spontaneous emission is inhibited in two directions in two dimensions, whereby light with a very narrow spectral width can be obtained with high efficiency.
(C) The optical element may have in the XY surface a refractive index distribution which is periodic in first, second, and third directions and include column-shaped first medium layers arranged in the shape of a triangular lattice or a honeycomb lattice, and a second medium layer formed between the first medium layers, for example. This optical element can form an incomplete photonic band in which spontaneous emission is inhibited in at least three directions in two dimensions, whereby light with a very narrow spectral width can be obtained with high efficiency.
The light-emitting layer may include an organic light-emitting material. Use of an organic light-emitting material widens the range of selection of materials in comparison with the case of using a semiconductor material or inorganic material, thereby enabling emission of light with various wavelengths.
Examples of the materials which can be used for each section of the light-emitting device according to the present invention are illustrated below. These materials are only some of available conventional materials. Materials other than the materials illustrated below may also be used.
Light-emitting Layer
The material for the light-emitting layer is selected from conventional compounds in order to obtain light with a predetermined wavelength. The material for the light-emitting layer may be either an organic compound or an inorganic compound. It is preferable to use an organic compound from the viewpoint of availability of a wide variety of compounds and deposition capability.
As examples of such organic compounds, organic compounds disclosed in Japanese Patent Application Laid-open No. 10-153967, such as aromatic diamine derivatives (TPD), oxydiazole derivatives (PBD), oxydiazole dimers (OXD-8), distyrylarylene derivatives (DSA), beryllium-benzoquinolinol complex (Bebq), triphenylamine derivatives (MTDATA), rubrene, quinacridone, triazole derivatives, polyphenylene, polyalkylfluorene, polyalkylthiophene, azomethine zinc complex, polyphyrin zinc complex, benzooxazole zinc complex, and phenanthroline europium complex can be given.
Conventional compounds disclosed in Japanese Patent Application Laid-open No. 63-70257, No. 63-175860, No. 2-135361, No. 2-135359, No. 3-152184, No. 8-248276, No. 10-153967, and the like may be used as the material for the organic light-emitting layer. These compounds may be used either individually or in combination of two or more.
As examples of inorganic compounds, ZnS:Mn (red region), ZnS:TbOF (green region), SrS:Cu, SrS:Ag, SrS:Ce (blue region), and the like can be given.
Optical Waveguide
The optical waveguide includes a layer which functions as a core and a layer which has a refractive index lower than that of the core and functions as cladding. Specifically, these layers include the light-transmitting section (core) and the insulating layer (cladding) in the light-emitting section, the core layer and the cladding layer in the waveguide section, the substrate (cladding), and the like. Conventional inorganic and organic materials may be used for the layers which make up the optical waveguide.
As typical examples of inorganic materials, TiO2, TiO2xe2x80x94SiO2 mixture, ZnO, Nb2O5, Si3N4, Ta2O5, HfO2, and ZrO2 disclosed in Japanese Patent Application Laid-open No. 5-273427, and the like can be given.
As typical examples of organic materials, various types of conventional resins such as thermoplastic resins, thermosetting resins, and photocurable resins can be given. These resins are appropriately selected depending on the layer formation method and the like. For example, use of a resin which can be cured by energy of at least either heat or light enables utilization of commonly used exposure devices, baking ovens, hot plates, and the like.
As examples of such substances, a UV-curable resin disclosed in Japanese Patent Application Laid-open No. 2000-35504 applied by the applicant of the present invention can be given. Acrylic resins are suitable as UV-curable resins. UV-curable acrylic resins excelling in transparency and capable of being cured in a short period of time can be obtained by using various types of commercially-available resins and photosensitizers
As specific examples of basic components of UV-curable acrylic resins, prepolymers, oligomers, and monomers can be given.
As examples of prepolymers and oligomers, acrylates such as epoxy acrylates, urethane acrylates, polyester acrylates, polyether acrylates, and spiroacetal-type acrylates, methacrylates such as epoxy methacrylates, urethane methacrylates, polyester methacrylates, and polyether methacrylates, and the like can be given.
As examples of monomers, monofunctional monomers such as 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, N-vinyl--2-pyrrolidone, carbitol acrylate, tetrahydrofurfuryl acrylate, isobornyl acrylate, dicyclopentenyl acrylate, and 1,3-butanediol acrylate, bifunctional monomers such as 1,6-hexanediol diacrylate, 1,6-hexanediol dimethacrylate, neopentyl glycol diacrylate, neopentyl glycol dimethacrylate, ethylene glycol diacrylate, polyethylene glycol diacrylate, and pentaerythritol diacrylate, and polyfunctional monomers such as trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, pentaerythritol triacrylate, and dipentaerythritol hexaacrylate can be given.
These inorganic and organic materials are illustrated taking into consideration only light confinement. In the case where the light-emitting section includes a light-emitting layer, a hole transport layer, an electron transport layer, and an electrode layer, and at least one of these layers functions as the core or cladding, materials for these layers may be employed as the material for the layers which make up the optical waveguide.
Hole Transport Layer
In the case of using an organic light-emitting layer in the light-emitting section, a hole transport layer may be optionally provided between the electrode layer (anode) and the light-emitting layer. As the materials for the hole transport layer, materials used as hole injection materials for photoconductive materials or materials used for a hole injection layer of organic light-emitting devices may be selectively used. The material for the hole transport layer is either an organic substance or an inorganic substance having a function of either hole injection or electron barrier characteristics. As specific examples of such substances, substances disclosed in Japanese Patent Application Laid-open No. 8-248276 can be given.
Electron Transport Layer
In the case of using an organic light-emitting layer in the light-emitting section, an electron transport layer may be optionally provided between the electrode layer (cathode) and the organic light-emitting layer. The electron transport layer has a function of transmitting electrons injected from the cathode to the organic light-emitting layer. The material for the electron transport layer may be selected from conventional substances. Substances disclosed in Japanese Patent Application Laid-open No. 8-248276 can be given as specific examples.
Electrode Layer
As the material for the cathode, electron injectable metals, alloys, electrically conductive compounds having a small work function (4 eV or less, for example), or mixtures of these may be used. Substances disclosed in Japanese Patent Application Laid-open No. 8-248276 can be given as specific examples of such electrode materials.
As the material for the anode, metals, alloys, electrically conductive compounds having a large work function (4 eV or more, for example), or mixtures of these may be used. In the case of using an optically transparent material for the anode, transparent conductive materials such as CuI, ITO, SnO2, and ZnO may be used. In the case where transparency is unnecessary, metals such as gold may be used.
In the present invention, the formation method for the optical element is not limited. Conventional methods may be used. Typical examples of such methods are given below.
(1) Lithographic Method
A resist layer is patterned by subjecting a positive or negative resist to exposure to ultraviolet rays, X-rays, or the like and development, thereby forming the optical element. Japanese Patent Applications Laid-open No. 6-224115 and No. 7-20637 disclose patterning technology using a resist formed of polymethylmethacrylate or a novolak resin.
Japanese Patent Applications Laid-open No. 7-181689 and No. 1-221741 disclose technology for patterning a polyimide using photolithography. Japanese Patent Application Laid-open No. 10-59743 discloses technology for forming an optical element formed of polymethylmethacrylate or titanium oxide on a glass substrate utilizing laser ablation.
(2) Method in which Regions Having Different Refractive Indices are Formed by Light Irradiation
Regions having different refractive indices are formed periodically in the optical waveguide section by irradiating the optical waveguide section with light with a wavelength which causes a change in the refractive index, thereby forming the optical element. As such a method, it is preferable to form the optical element by forming a layer of a polymer or a polymer precursor and polymerizing part of the layer by irradiation or the like, thereby periodically forming the areas having different refractive indices. This type of techniques are disclosed in Japanese Patent Applications Laid-open No. 9-311238, No. 9-178901, No. 8-15506, No. 5-297202, No. 5-39480, No. 9-211728, No. 10-26702, No. 10-8300, No. 2-51101, for example.
(3) Stamping Method
The optical element is formed by hot stamping using a thermoplastic resin (Japanese Patent Application Laid-open No. 6-201907), stamping using an UV curable resin (Japanese Patent Application Laid-open No. 2000-35504), or stamping using an electron beam curable resin (Japanese Patent Application Laid-open No. 7-235075), or the like.
(4) Etching Method
The optical element is formed by selectively removing and patterning a thin film using lithography and etching technologies.
The method for forming the optical element is described above. In summary, the optical element consists of at least two areas having different refractive indices. The optical element may be formed by a method of forming the two areas using two types of materials having different refractive indices, a method of forming the two areas having different refractive indices by modifying part of one type of material, or the like.
Each layer of the light-emitting device may be formed using a conventional method. A deposition method suitable for each layer of the light-emitting device is appropriately selected depending on the materials therefor. As specific examples of such a method, a vapor deposition method, spin coating method, LB method, ink jet method, and the like can be can be given.